This application claims the priority benefit of Taiwan application Ser. No. 89115832, filed Aug. 7, 2000.
1. Field of the Invention
The present invention relates to an optical fiber grating. More particularly the present invention relates to an electric optical fiber grating that can switch central wavelength.
2. Description of the Related Art
With the development of high-density wavelength multiplex optical transmission technology, it is very important to add or to drop in required signal wavelengths at certain junctions. In order to increase the adaptability of transmission capacity, it is very important to design for wavelength restructuring.
A Bragg optical fiber grating provides a fiber with periodic variance of refractive index at the core of the fiber. This phenomenon can be achieved by irradiating an interference pattern on the optical fiber. The interference pattern can be obtained by causing the interference between two ultra-violet rays or directly causing interference on the ultra-violet ray through a mask. Due to the optical sensitivity of the optical fiber, the refractive index periodically varies.
After light beam is incident into an optical fiber grating, only if a wavelength of the incident light satisfies the Bragg""s condition, where the wavelength equal to the central wavelength of the optical fiber grating, the incident light with the specific wavelength can be reflected. The central wavelength of the reflection of the optical fiber grating is equal to a quantity of two times of arithmetic product of the equivalent refraction index and the spacing of refraction changes. The other incident light with wavelengths without satisfying the Bragg""s condition are not reflected and they all pass the optical fiber grating.
When the temperature exerting on the optical fiber grating has changed, the temperature effect would cause a shift on the central wavelength of the optical fiber grating. When a wavelength of 1550 nm is operated on, the wavelength temperature coefficient is approximately 0.012 nm/xc2x0 C. That is, when temperature changes 1xc2x0 C., the central wavelength of an optical fiber grating changes 0.012 nm.
In addition, when the stress applied to the optical fiber grating due to external changes, the change of the stress also affects the equivalent refraction index and refraction spacing length, and causes shifting of the central wavelength of the optical fiber grating. When a wavelength of 1550 nm is operated on, the wavelength strain coefficient is approximately 1.2 nm/millistrain.
Owing to the two characteristics described above, there are different considerations in the application of optical fiber grating. For example, a technology disclosed by US patent (U.S. Pat. No. 5,694,503) fixes the optical fiber grating on a ceramic substrate with a negative temperature thermal expansion coefficient. Other examples, such as the technologies disclosed by U.S. patent (U.S. Pat. No. 5,841,920) and World Intelligence Property Organization patent (WO9827446), uses a structure formed by two components with very different thermal expansion coefficients to produce a negative temperature thermal expansion effect. The above technologies are all applied to fixed wavelengths. They use a ceramic substrate with a negative temperature thermal expansion coefficient or a structure formed by two components with very different thermal expansion coefficients to produce stress to compensate for the shifting of the central wavelength of an optical fiber grating caused by the change of temperature. In this way, the shifting of the central wavelength of an optical fiber grating produced when temperature changes, is reduced, which has an equivalent effect on the reduction of the wavelength temperature coefficient.
In the application of adjusting central wavelengths of an optical fiber grating, a technology that is disclosed by U.S. patent (U.S. Pat. No. 5,812,711) is directed to adjusting the wavelength of an optical fiber grating by way of magnetic strain. Another technology that is disclosed by U.S. patent (U.S. Pat. No. 5,469,520) is directed to adjusting the wavelength of an optical fiber grating by way of compression strain. In addition, a technology that is disclosed by European patent (EP0867736) is directed to adjusting the wavelength of an optical fiber grating by way of heating.
With an increase of the wavelength multiplex channels, the space length between channels has gradually reduced from 1.6 nm to 0.4 nm. The accuracy of the corresponding central wavelength also needs to reach xc2x10.025 nm. If the temperature control is adopted as a way to adjust wavelength, it requires long-term power consumption. Adjusting wavelength by way of piezoelectric material (e.g. PbZrTiO3, PZT) strain also requires long-term power consumption. While magnetic strain only requires power consumption during wavelength adjustment, it still requires consideration of the wavelength temperature coefficient of the optical fiber grating and on the thermal expansion coefficient of the magnetic material itself. As to the adjustment by way of optical fiber vertical strain, the accuracy of the adjustment needs to reach the resolution of several micrometers, therefore it requires great accuracy of position during fabrication.
Up to now, the above structures for adjusting central wavelength did not take into consideration the effect of temperature on wavelength shifting. That is, after these optical fiber gratings are adjusted to certain wavelengths, the situation of central wavelength shifting occurs due to the influence of temperature.
Therefore the present invention provides an electric optical fiber grating filter that can switch central wavelength, which uses a bimetal plate, an elastic structure, and an solenoid to form an optical fiber grating that can switch central wavelength and prevent the central wavelength from shifting.
The present invention provides an electric optical fiber grating filter that can switch central wavelength. The grating does not need to monitor the temperature and feedback control at all times. Therefore, it does not consume power. At the same time, it can adjust wavelength and has the advantage of not being sensitive to temperature.
An electric optical fiber grating filter that can switch central wavelength comprises at least a bimetal plate, an optical fiber, a spring, a screw, a mobile structure, and an solenoid. The bimetal plate includes a first metal plate and a second metal plate joined together, and the thermal expansion coefficient of the first metal plate is smaller than that of the second metal plate. An optical fiber grating is provided in the optical fiber. The optical fiber is fixed along the surface of the first metal plate. Because of the difference in the thermal expansion coefficients of the first metal plate and the second metal plate, the bimetal plate bends when temperature changes, causing change of the central wavelength of the optical fiber grating. The change in the wavelength can be used to compensate for the shifting of the wavelength of the optical fiber grating caused by the change of temperature. A first end of the spring is connected to the second metal plate. The screw is connected to a second end of the spring to adjust the number of spring coils and to optimize the compensation for the shifting of the central wavelength of the optical fiber grating caused by the change in temperature. The screw is installed on the mobile structure and the solenoid is connected to the mobile structure to stretch the spring to a first position or to compress the spring to a second position so as to cause the bimetal plate to produce the bending changes. In this way, the central wavelength of the optical fiber grating can switch between two predetermined values.
Another electric optical fiber grating filter that can switch central wavelength is provided by the present invention. The filter comprises at least a bimetal plate, an optical fiber, an elastic structure, a mobile structure and an solenoid. The pair of metal plates include a first metal plate and a second metal plate joined together, and the thermal expansion coefficient of the first metal plate is smaller than that of the second metal plate. An optical fiber grating is provided in the optical fiber. The optical fiber is fixed along the surface of the first metal plate. Because of the difference in the thermal expansion coefficients of the first metal plate and the second metal plate, the bimetal plate bend when temperature changes, and thus cause the central wavelength of the optical fiber grating to change. The change in the wavelength can be used to compensate for the shifting of the wavelength of the optical fiber grating caused by the change in temperature. A first end of the elastic structure is connected to the second metal plate. A second end of the elastic structure is installed on the mobile structure. An solenoid is connected to the mobile structure to stretch the elastic structure to a first position or to compress it to a second position so as to cause the bimetal plate to produce the bending changes. In this way, the central wavelength of the optical fiber grating can switch between two predetermined values.
The present invention provides a method for switching the central wavelength of an electric optical fiber grating filter that can switch central wavelength. The above described optical fiber grating filter includes a bimetal plate, an optical fiber, which is provided with an optical fiber grating and is fixed along the surface of the bimetal plate, an elastic structure, one end of which is connected to the bimetal plate, and an solenoid, which is connected to the other end of the elastic structure. The above described method includes at least: using the solenoid to stretch the elastic structure to a first position or compress it to a second position so as to cause the bimetal plate to bend. In this way the central wavelength of an optical fiber grating can be switched.